about the last 2 problem sets

Problem set 3 and problem set 4 are reletively easier for me. One problem I encountered during problem 3 is I forgot the formula of the sum of the propotionally increasing/decreasing series, which is studied in high school, so I have to go to an old text book and check. This experience tells me that the old stuff I studied before, no matter how detail or invisible it seems, it may still play a valuable role in my current study, so I better review my old notes and text books to get though the course more smoothly. The problem set 4 is not hard too, because I can somehow recall stuff I learned from last year's cs courses, which gives me a small advantage. So it tells me that keep my old knowlage can benefit my new study by making my studing subject more familiar so I can catch up easier.